feat(spm): add FFA_RX_ACQUIRE forwarding in SPMD

Add FF-A v1.1 FFA_RX_ACQUIRE ABI forwarding to SPMD.
RX acquire interface is used by Hypervisor to acquire ownership of a
VM's RX buffer from SPMC when it needs to deliver a message to the VM.

Change-Id: I5f57240a9c9e94eb696a5a394ec0644170380026
Signed-off-by: Federico Recanati <federico.recanati@arm.com>
diff --git a/services/std_svc/spmd/spmd_main.c b/services/std_svc/spmd/spmd_main.c
index 27a8382..649fe9f 100644
--- a/services/std_svc/spmd/spmd_main.c
+++ b/services/std_svc/spmd/spmd_main.c
@@ -755,6 +755,7 @@
 	case FFA_NOTIFICATION_INFO_GET:
 	case FFA_NOTIFICATION_INFO_GET_SMC64:
 	case FFA_MSG_SEND2:
+	case FFA_RX_ACQUIRE:
 #endif
 	case FFA_MSG_RUN:
 		/*